A state appeals court says Marin County's latest land-use plan failed to assess the effects of proposed development on the habitat of threatened species of coho salmon and steelhead trout.

The general plan adopted by county supervisors for unincorporated areas in 2007 also lacked measures to reduce the impact of construction on the fish and their habitat, the First District Court of Appeal in San Francisco said Wednesday. The court ordered the county to conduct a new environmental study and invite more public comment.

The ruling protects the last remaining wild population of California coho coastal salmon, said the Turtle Island Restoration Network, which challenged the plan in court.

"We hope that after this decision, county supervisors are ready to work together so we can save these species from extinction," said Todd Steiner, executive director of the organization's salmon-protection program.

Deputy County Counsel David Zaltsman said he believes the county can comply with the ruling and prepare a new report. He said the county has not approved any new development in the affected watershed and will impose restrictions to protect fish habitat when owners of any of the 207 vacant parcels of land submit construction plans.

The dispute involves the San Geronimo Valley watershed, which covers 9.3 square miles in west Marin County. The 2007 plan predicted a 20 percent increase in housing units and a 40 percent increase in other development in the area by 2030.

The plan recognized the potential impact of construction on waterways and called for measures to protect and restore fish habitat. But the court said the county plan, and an accompanying environmental study, failed to evaluate the degree to which "such construction is likely to affect the streams," or to assess "the likely cumulative effects of a widespread build-out."

The plan also "defines no specific measures to be taken to reduce the impact of build-out on the threatened fish species," said Justice Stuart Pollak in the 3-0 ruling. He said Marin County is participating with five other coastal counties in a habitat-protection program, but has not committed itself to following the program's recommendations.